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SubjectRe: [BUG] ptraced process waiting on syscall may return kernel internal errnos
Date
Oleg and I were just discussing this issue in relation to other problems.
We established that it is never safe to clear TIF_SIGPENDING on another
thread. But I hadn't really thought through that it's sometimes not safe
to clear your own TIF_SIGPENDING either. That is, any time you are not
positive you cannot be in a syscall that will return a -ERESTART* code.
(I had the ptrace_stop case lurking in the back of my mind but hadn't
considered how it would really come up.)

I have a general recollection of thinking that dequeue_signal could only
be called on current and that it mattered somehow. But aside from
avoiding recalc_sigpending, and kernel threads with notifier_mask set, I
can't see off hand what it is. I won't testify that I think signalfd is
necessarily on safe ground, though.


Thanks,
Roland

---

[PATCH] Restrict clearing TIF_SIGPENDING

This patch should get a few birds. It prevents sigaction calls from
clearing TIF_SIGPENDING in other threads, which could leak -ERESTART*.
It fixes ptrace_stop not to clear it, which done at the syscall exit
stop could leak -ERESTART*. It probably removes the harm from
signalfd, at least assuming it never calls dequeue_signal on kernel
threads that might have used block_all_signals.

Signed-off-by: Roland McGrath <roland@redhat.com>
---
kernel/signal.c | 40 ++++++++++++++++++++++++----------------
1 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index acdfc05..dc5797c 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -105,7 +105,11 @@ static int recalc_sigpending_tsk(struct
set_tsk_thread_flag(t, TIF_SIGPENDING);
return 1;
}
- clear_tsk_thread_flag(t, TIF_SIGPENDING);
+ /*
+ * We must never clear the flag in another thread, or in current
+ * when it's possible the current syscall is returning -ERESTART*.
+ * So we don't clear it here, and only callers who know they should do.
+ */
return 0;
}

@@ -121,7 +125,9 @@ void recalc_sigpending_and_wake(struct t

void recalc_sigpending(void)
{
- recalc_sigpending_tsk(current);
+ if (!recalc_sigpending_tsk(current))
+ clear_thread_flag(TIF_SIGPENDING);
+
}

/* Given the mask, find the first available signal that should be serviced. */
@@ -385,7 +391,8 @@ int dequeue_signal(struct task_struct *t
}
}
}
- recalc_sigpending_tsk(tsk);
+ if (likely(tsk == current))
+ recalc_sigpending();
if (signr && unlikely(sig_kernel_stop(signr))) {
/*
* Set a marker that we have dequeued a stop signal. Our
@@ -1580,8 +1587,9 @@ static void ptrace_stop(int exit_code, i
/*
* Queued signals ignored us while we were stopped for tracing.
* So check for any that we should take before resuming user mode.
+ * This sets TIF_SIGPENDING, but never clears it.
*/
- recalc_sigpending();
+ recalc_sigpending_tsk(current);
}

void ptrace_notify(int exit_code)
-
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